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linux初始化的时候使用 init_task 进程, 该进程是静态创建的, 别的进程都是通过该进程克隆出来的。
静态创建的含义是 current 指向 init_task 进程, 并且内核堆栈使用的是 init_task 的堆栈.下文以linux4.16.3为例描述 init_task current 如何撑起这个艰难的任务的。
current_task定义如下，都是每个cpu一个：
DEFINE_PER_CPU(struct task_struct *, current_task) = &init_task;EXPORT_PER_CPU_SYMBOL(current_task);DECLARE_PER_CPU(struct task_struct *, current_task);
current_task的使用方法：
static __always_inline struct task_struct *get_current(void){ return this_cpu_read_stable(current_task);}#define current get_current() init_task定义如下:
struct task_struct init_task{ ... ... .stack = init_stack, ... ...} init_stack 为 init_task 的堆栈, 地址为
#define INIT_TASK_DATA(align) \ . = ALIGN(align); \ VMLINUX_SYMBOL(__start_init_task) = .; \ VMLINUX_SYMBOL(init_thread_union) = .; \ VMLINUX_SYMBOL(init_stack) = .; \ *(.data..init_task) \ *(.data..init_thread_info) \ . = VMLINUX_SYMBOL(__start_init_task) + THREAD_SIZE; \ VMLINUX_SYMBOL(__end_init_task) = .;
还有一个有用的宏定义
#define TOP_OF_INIT_STACK ((unsigned long)&init_stack + sizeof(init_stack) - \ TOP_OF_KERNEL_STACK_PADDING)
堆栈的关联
__visible DEFINE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw) = { .x86_tss = { /* * .sp0 is only used when entering ring 0 from a lower * privilege level. Since the init task never runs anything * but ring 0 code, there is no need for a valid value here. * Poison it. */ .sp0 = (1UL << (BITS_PER_LONG-1)) + 1,#ifdef CONFIG_X86_64 /* * .sp1 is cpu_current_top_of_stack. The init task never * runs user code, but cpu_current_top_of_stack should still * be well defined before the first context switch. */ .sp1 = TOP_OF_INIT_STACK, // sp1 设置堆栈#endif#ifdef CONFIG_X86_32 .ss0 = __KERNEL_DS, .ss1 = __KERNEL_CS, .io_bitmap_base = INVALID_IO_BITMAP_OFFSET,#endif },#ifdef CONFIG_X86_32 /* * Note that the .io_bitmap member must be extra-big. This is because * the CPU will access an additional byte beyond the end of the IO * permission bitmap. The extra byte must be all 1 bits, and must * be within the limit. */ .io_bitmap = { [0 ... IO_BITMAP_LONGS] = ~0 },#endif};EXPORT_PER_CPU_SYMBOL(cpu_tss_rw); 内核启动 head_64.S 里面
.code64 .globl startup_64startup_64: ... ... /* Set up the stack for verify_cpu(), similar to initial_stack below */ leaq (__end_init_task - SIZEOF_PTREGS)(%rip), %rsp ... ... /* Setup a boot time stack */ movq initial_stack(%rip), %rsp
如上汇编都是将内核的堆栈设置到了init_task对应的内核堆栈上, 其中 initial_stack 的定义如下
GLOBAL(initial_stack) /* * The SIZEOF_PTREGS gap is a convention which helps the in-kernel * unwinder reliably detect the end of the stack. */ .quad init_thread_union + THREAD_SIZE - SIZEOF_PTREGS
所以当 current 指向 init_task, 并且 init_task 的 堆栈为当前的 rsp 的时候, 就可以认为当前的执行进程为 init_task
最后看看 init_task 如何克隆别的进程
void __ref rest_init(void){ pid = kernel_thread(kernel_init, NULL, CLONE_FS); =>pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) { return _do_fork(flags|CLONE_VM|CLONE_UNTRACED, (unsigned long)fn, (unsigned long)arg, NULL, NULL, 0); =>long _do_fork(unsigned long clone_flags, unsigned long stack_start, unsigned long stack_size, int __user *parent_tidptr, int __user *child_tidptr, unsigned long tls) { p = copy_process(clone_flags, stack_start, stack_size, child_tidptr, NULL, trace, tls, NUMA_NO_NODE); =>struct task_struct *copy_process( unsigned long clone_flags, unsigned long stack_start, unsigned long stack_size, int __user *child_tidptr, struct pid *pid, int trace, unsigned long tls, int node) { struct task_struct *p = dup_task_struct(current, node); // 这时 current 为 init_task, 所以克隆的是 init_task 的信息 } } } pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);} 内核中current实现
https://www.cnblogs.com/tsecer/p/10487596.html一步一步学linux操作系统: 11 进程数据结构_task_struct字段_内核栈
https://blog.csdn.net/leacock1991/article/details/106977793Linux操作系统学习笔记（五）进程的核心——task_truct
https://ty-chen.github.io/linux-kernel-task-struct/